What Car? Green Awards 2009 - Luxury cars

Best buy Audi A8 2.8 V6 FSI SESettling back against the sumptuous leather upholstery of your luxury car might not feel as comfortable as it sounds if your conscience isn't at rest.

Luxury cars are big cars, after all, and made heavier still by all the tricks and toys that are loaded into them. Weight is public enemy number one when it comes to efficiency and emissions, so where can a captain of industry look if he wants to tread lightly and still carry a big stick?

Step forward, for the second year in a row, the aluminium-bodied Audi A8, which emits less than 200g/km of CO2 and that can manage an impressive 34.0mpg on average.

Number crunchingThese are produced by a smooth V6 petrol engine, too, so if you're swinging by the school to pickup the children, you don't have to worry about any of those nasty particulates blowing into the playground.

It also means that you don't have to pay the 3% company car tax penalty levied on diesel engines, and NOx emissions are also very low.

Yes, the new BMW 730d does have a lower CO2 output and better average economy, but because it's a diesel, it comes with higher NOx and particulates as a result – and it just isn't the great luxury car that we were hoping it would be.

So plush and so greenThe Audi's really ticking all the right boxes so far, then – but don't think for a second that you need to make any sacrifices by owning it. Yes, the A8 is green as far as luxury cars go, but it doesn't achieve it at the expense of space, comfort or driveability.As well as having one of the most beautifully made cabins around, the interior of the A8 has masses of room and there's more than enough space in the boot for a family's holiday luggage.

All the features you would expect from a car in this category come as standard. You get leather upholstery, of course, as well as electrically adjustable seats, climate and cruise controls, and satellite-navigation all included in the price.

The ride is composed, thanks to its air suspension, and the car is also quiet at speed – barely a grumble is heard from the engine, even under hard acceleration. Drive it on a motorway and you might notice some wind noise, but you still won't have to raise your voice to make yourself heard.

It might not sound like much compared with rivals, but we think the 2.8-litre engine has more than enough poke to get you going. It won't give you a face-lift, but it still manages to sprint to 60mph in 8.0 seconds and reach a top speed of 149mph.

Buying usedGet a year-old A8 and you can easily chop £20,000 off the price for a car that may have done only 5000 miles. You could find an independent or private sale, but franchised dealers have most on their approved-used schemes.

Best buy £50,000-£70,000Mercedes-Benz S320 CDI Price: £55,590 Target Price: £50,554 CO2: 220g/km NOx: 0.198g/km Particulate emissions: 0.003g/km Average fuel economy: 34.0mpgGreen verdict: It might not be quite as clean as the Audi, but the Mercedes is even more luxurious to live with